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Stainless Steel Terminlogy

Ray C

Staff member
Also, sometimes you'll see the term "TGP" on round stock. It means Turned, Ground, Polished.



Active User
H-M Supporter-Premium
The 300 series stainless steels are the most common stainless steels we hobbyists happen across. I don't know much about the 400 series which are known as "martensitic" stainless steels because of the molecular structure. The 300 series, 303, 304, 316, 321, etc. are "austenitic" again because of the molecular structure. The 300 series are not magnetic, but the 400 series are magnetic which means one can use a magnet to identify the general type. The 400 series is used for kitchen knives because it will hold an edge, well not like carbon steel, but stay sharper than other stainless steels.

Because the 300 series is what it is it work hardens during roll forming in manufacture, it's nasty stuff to roll and form, hot or cold. As anyone who has ever tried to turn 304 in a lathe can testify the stuff is "sticky+stringy" and mean to cut. (303 is the best machining stainless but my not be welded) The mills making the stainless have related problems; they have to hook chains to the end edges of plate to help pull it through the rolls etc. In between sizing and rolling operations the mills have to thermally anneal the stainless in order to actually work the stuff without destroying the forming equipment. The mill also uses salts and acids to free the scale from the product during and at the end of the manufacturing process. This is another reason stainless is expensive.

During the last sizing and finishing operations the stainless is thermally annealed one last time and the scale removed, most often with acid, hence the term "pickled". There are some other preparations used to render the stainless products ready for different user markets, but "pickled and annealed" is the most common. Geoff Morgan